Electronic balances in many cases are calibrated by means of an internal calibration weight. To perform a calibration, a calibration weight of a specifically defined mass is brought into force-transmitting contact with the force-transmitting device that is arranged in a force-measuring cell of a balance, whereupon a reference value is determined. Based on this reference value, further weighing parameters of the balance can be adjusted. After the calibration has been performed, the contact between the calibration weight and the force-transmitting device is released again, and the calibration weight is secured in a rest position. In the foregoing process, the calibration weight is moved from a rest position into a calibrating position and back to the rest position by a transfer mechanism which includes at least one lifting element and a drive source. In the calibrating position, the calibration weight is in force-transmitting contact with the force-transmitting device; in the rest position there is no force-transmitting contact.
The known state of the art includes different kinds of lifting elements and realizations of calibration weight arrangements.
A calibration weight, as disclosed in commonly-owned U.S. Pat. No. 5,148,881 to Leisinger, is moved vertically by wedges arranged in pairs that slide horizontally against each other, whereby the calibration weight is brought into force-transmitting contact with the force-transmitting device of the balance. This lifting element is driven by a motor through a spindle that is connected to the wedges.
A likewise vertical lifting and lowering of a calibration weight is achieved by a device that is described in commonly-owned U.S. Pat. No. 6,194,672 B1 to Burkhard. The weight rests on a holder which is moved by an electrically driven lifting element.
German Gebrauchmuster 203 18 788 U1 describes a monolithic calibration weight that is raised and lowered by a ramp-like lifting element which is driven by a linear drive and performs a kind of inclined parallel movement.
In many balances, the calibration weight arrangement and the force-transmitting device are arranged behind one another in the manner disclosed in the Burkhard '672 patent. However, the calibration weight can also be split up, for example into two calibration weights that are attached laterally to the force-transmitting device like the calibration weights disclosed in commonly-owned U.S. Pat. No. 5,866,854 to Emery, which have the shapes of circular cylinders. The two identical weights are arranged on two opposite sides of the force-transmitting device. Two different mechanisms for moving the calibration weights are described. In the first case, the calibration weight which comprises a guide pin rests on a calibration weight holder that is configured as a support cradle. To perform a calibration, the calibration weight holder which is pivoted on one side is tilted downwards, whereby the calibration weight is lowered onto two calibration weight receivers below the calibration weight holder, which have the shape of rods or levers and are connected to the force-transmitting device. In a second variant, the weight in its rest position lies on a calibration weight holder that is arranged between the calibration weight receivers that are connected to the force-transmitting device. To perform a calibration, a vertical downward movement of the calibration weight holder brings the calibration weight into contact with the calibration weight receivers.
In general, the aforementioned lifting elements are driven by small servomotors. It is a disadvantage in using servomotors that they use a relatively large amount of space in the force-measuring cell of the balance, whereby the size of the force-measuring cell itself as well as of the balance is increased unnecessarily.
Especially in electronic balances of high sensitivity, the weighing result is influenced or even changed by electrostatic charges and interactions. The servomotors used to drive the transfer mechanisms contain electrically non-conductive gearbox parts which in the course of their operation produce electrostatic fields through friction. The resultant electrostatic fields, and also electromagnetic fields, are strong enough to influence the weighing result, particularly in balances of high sensitivity.
An alternative drive source associated with a balance is shown in JP 59090031 A, which discloses the use of a shape memory alloy (SMA) used in connection with a balance for the determination of the volume and the specific gravity of a sample. The shape memory alloy has the shape of a spiral-wound wire connected to hooks at both ends. The upper hook is connected to a balance, and the sample is suspended on the lower hook. The weight of the sample is first measured in air. In the following step, the SMA wire is exposed to infrared radiation and thereby indirectly heated. The heating causes an axial contraction of the SMA material which has the effect that the sample, which remains fastened to the hook during the entire procedure, is lowered into a water bath that has been set in place, whereupon the weight of the sample under water is determined.
The shape memory alloy is used in this case in the shape of a spiral spring that can vary its length and is arranged as an extension or as a substitute for the weighing pan. This spring is freely suspended from the balance that is arranged above the spring and the water bath. The movement of the spring is not guided by constraints, which means that the sample is free to rotate, tilt or swing. The critical issue in measurements of this kind is that the sample can be completely submerged in the liquid; the speed is irrelevant in this process as long as no liquid escapes from the container.
Almost always, the calibration weight arrangements of the known prior art comprise relatively large drive sources. An improvement of the calibration weight arrangement therefore requires in particular an optimization and miniaturization of the drive source of the transfer mechanism. The drive source needs to be very small, compact, and flexible in the ways it can be used.